The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) is designed to evaluate the role of isopycnal (mostly horizontal) mixing along the steeply tilted density surfaces of the Southern Ocean, and at the same time to determine the magnitude of diapycnal (mostly vertical) mixing processes that have the potential to transform water properties at mid-depth. Together these two processes are key contributors to the global meridional overturning circulation. DIMES carried out an extensive sampling program including acoustically-tracked sub-surface floats, a mid-depth tracer release, a mooring array and water property surveys. Now that the bulk of DIMES data collection has taken place, the objective of this project is to make optimal use of the DIMES observations by employing them as constraints for an ocean general circulation model, the Southern Ocean State Estimate (SOSE), which is run using a 4-dimendional variational data assimilation technique. SOSE will provide best estimates of the background conditions in the DIMES research domain, which is vital for the DIMES float and tracer analysis. SOSE fields will be closely compared with DIMES and other independent observations to provide an assessment of uncertainties in SOSE, as well as uncertainties in the DIMES observations and in its derived estimates of Lagrangian mixing. SOSE fields will reveal the details of isopycnal mixing and oceanic heat transport in the DIMES domain and throughout the Southern Ocean. Lessons learned in the DIMES domain about isopycnal mixing will be extended to the entire Southern Ocean to evaluate spatial variations in isopycnal mixing and its sensitivity to local bathymetry, wind, and buoyancy forcing. SOSE fields will also be analyzed to evaluate the strength of meridional overturning circulation, and to assess poleward heat transport across the ACC and towards the climatically sensitive ice shelves surrounding Antarctica.
Broader Impacts: Research conducted for this project using SOSE will contribute to the broader multi-institution, multi-PI efforts to interpret data collected as part of DIMES, with the ultimate goal of developing a better understanding of along-isopycnal and diapycnal mixing processes in the ocean. Output from SOSE will to be made publicly available to be used for analysis by DIMES-affiliated research groups and by other interested researchers. The state estimate setup (forward and adjoint model) will be at the DIMES research groups' disposal for additional runs in support of their science goals. Results from DIMES will ultimately lead to better physical understanding of these mixing processes and suggest improvements in the mixing parameterizations of numerical models used for future climate projections. In addition, the project will contribute to the training of one graduate student who currently holds a NASA Earth System Science graduate fellowship and will provide support for a post-doctoral researcher, who will develop his skills in assimilation and model analysis.
This project is a contribution to the U.S. CLIVAR (Climate Variability and Predictability) Program.
